The Medium Access Control (MAC) data communication protocol layer provides addressing and channel access control mechanisms for several network nodes to communicate within a multipoint network. Such a network can be a local area network (LAN) as in a wired or wireless network. For a wireless network, the MAC layer manages and maintains communications between wireless communication stations by coordinating access to a shared wireless (e.g., radio frequency) channel utilizing MAC protocols for communications over the wireless channel.
In a contention-based MAC protocol without channel sensing, all stations contend for the access to a shared channel, wherein a packet transmission is successful when only one station attempts to transmit the packet. When multiple nodes attempt transmitting packets over the shared channel simultaneously, packet collisions occur.
In a Pure Aloha MAC protocol for packet radio networks, a station transmits a packet over the channel whenever a new packet arrives into the transmission queue of that station. Packet collision occurs if more than one station transmits at the same time, resulting in a retransmission of the packet at some time in the future, independent of other stations. As a result, the communication system throughput of a network using a Pure Aloha MAC protocol is about 18%.
FIG. 1 shows a variation of the Pure Aloha MAC protocol, know as a Slotted Aloha protocol. In the example protocol 1 shown in FIG. 1, time is divided into slots 2, and each slot 2 comprises a fixed period of time defined to be the duration required to transmit a packet 3. Stations are forced to start transmission of packets only at slot boundaries. Therefore, vulnerability to packet collisions (i.e., the vulnerability period) is reduced to the duration of one slot. A packet transmission is successful if and only if, exactly one packet is scheduled in a slot. In comparison to a Pure Aloha protocol, the vulnerability period is reduced by half, thus, doubling the throughput of a Slotted Aloha relative to a Pure Aloha. As such, the system throughput of a Slotted Aloha protocol is 36%.
However, the Slotted Aloha MAC protocol has several disadvantages. Each slot should be long enough to accommodate the largest size packet. If packets are of variable length, then unused slot durations lead to channel bandwidth waste or lower channel utilization. Even if a packet is transmitted successfully, the transmitting station (the sender) has no information about the success of the transmission. Thus, an explicit acknowledgement (ACK) is required from a receiving station (the receiver) in a different time slot. This further consumes channel bandwidth. Further, upon receiving the ACK, the transmitting station must acknowledge such receipt to the receiving station, further consuming channel bandwidth. In addition, the latency from the instant a data packet is transmitted from a transmitting station to the instant following receipt of a corresponding ACK from a receiving station can be quite large. Such a large latency is a disadvantage for transmission of delay sensitive packets.
FIG. 2 shows a Modified Slotted Aloha protocol 5, wherein the size of a slot 6 is increased (compared to slot 2 in FIG. 1 for the Slotted Aloha), so that an ACK 7 is transmitted within the same slot 6 as the data packet 8. The size of each slot 6 is large enough for transmission of the largest size data packet, an ACK 7, and an Inter-Frame Space (IFS) duration 9 for channel switching and other overhead. Transmitting the ACK 7 from the receiving station in the same slot as the transmission of the corresponding packet 8 from the transmitting station allows prompt invocation of retransmissions, if necessary. However, there remains channel waste due to variable length packet size. This is because any unused slot cannot be utilized for transmitting other packets, and because packet transmission is scheduled at the start of a slot boundary.